Usually, a valve disc and a lantern ring of a stop valve are both made of metal materials. When the stop valve is in a cut-off state, a large fastening torque is needed to provide a good sealing performance to a sealing surface between the valve disc and the lantern ring, but the large fastening torque tends to cause abrasion to the sealing surface between the valve disc and the lantern ring, thus resulting in a leakage inside the stop valve.